1. Field of the Invention
This invention relates to lifting devices, specifically to improvements in such devices that are employed to lift watercraft, for example boats and seaplanes.
2. Description of the Related Art
Users of watercraft have need to lift their watercraft from the water, for example for maintenance or in preparation for land transportation of the watercraft. Equally, watercraft users have need to lower their watercraft into the water, for example for launching or simply for flotation at dock. Users have heretofore employed a number of devices for such lifting and lowering.
Typical of many such devices, U.S. Pat. No. 4,895,479, issued to Michaelsen et al., specifies a lift for watercraft having raised and lowered positions, comprising a substantially horizontal rectangular base for submersion in the water. In its preferred embodiment, the base, having a front, a back and two sides, is fixed and two pair of upwardly extending booms are each pivotally connected at their lower end to the base at its corners. The members of each pair of booms are rigidly connected to each other in parallel by cross supports. Two substantially horizontal arms are each pivotally connected to one member of each pair of booms at the upper end of the boom, so that each arm pivotally connects to one member of one pair of booms at one end of the arm and to one member of the other pair of booms at the other end of the arm. Two connected, collapsible parallelograms are thereby formed, one on each side of the base, the corners of each parallelogram comprising the upper and lower pivot points of the front and rear booms on one side of the base. The pivotally connected arms lie in a plane, which is lowered as the parallelograms are collapsed and is raised when the parallelograms are expanded. A watercraft supported by support means connected to the arms in that plane may thereby be lowered and raised as the parallelograms are respectively collapsed and expanded.
In order to raise the watercraft in Michaelsen, as in most such devices, the parallelogram is forceably expanded by employment of an expanding actuator, typically a hydraulic cylinder. In Michaelsen, the base end of a hydraulic jack cylinder is connected centrally to a horizontal coplanar cross member of the frame disposed intermediate of the points of connection of the pairs of pivoting booms. The plunger of the jack is connected to a cross shaft journaled in brackets intermediate of the ends of that pair of pivoting booms that normally (in the lowered position) forms an obtuse angle in the parallelogram, thereby achieving a large angular swing of the lift arms for a given travel of the jack's plunger. Significantly, because raising the lift requires a vertical force component at all times, this actuator arrangement limits the extent to which this lift may be lowered, because, even at the lift's lowest level, a vertical component is required between the base of the jack and the distal end of its plunger in order for the expansion of the jack to have the effect of raising the lift.
In such devices, when the actuator has caused the watercraft to be raised to the desired position, a locking means is employed to maintain the craft in the raised position. In Michaelsen, locking is provided by a pawl mounted on the jack cylinder adjacent to its projecting plunger, the pawl having a nose fittable in spaced slots of the plunger and biased into the slots so that it automatically locks the lift in subsequent raised positions.
In U.S. Pat. No. 3,021,965 to Harvey, locking is provided by a locking and bracing assembly, comprising a rod or brace pivotally connected to a side of the base intermediate of the pivotal connection points of two collaterally corresponding members of the pairs of booms. The locking and bracing assembly further comprises a means for the rod lockably to engage that member which is over-center when the device is in the raised position, restraining that member against the force of gravity, thereby maintaining the device in the raised position.
U.S. Pat. No. 5,275,505 to Wilcox describes a locking mechanism having a clamping ring which is sleeved on the piston rod and is biased into locking position by a compression spring engaging a piston in a locking cylinder. The locking cylinder is selectively supplied with pressurized water to compress the spring and thereby release the clamping ring. These and other methods of locking a piston driven system against a force, well-known to practitioners in this and analogous arts, are widely employed in watercraft lift design.
U.S. Pat. No. 5,184,914, issued to the inventor of the present invention and which is incorporated herein by reference, takes a different approach to locking. The inventions heretofore described provide locking by restraining, by various methods, the collapse of partially expanded parallelograms. In '914, in contrast, the actuator expands the parallelogram over-center, so that initially obtuse angles of the parallelogram become acute and initially acute angles become obtuse. In the over-center raised position, preferably one to ten degrees from vertical, a locking means is provided by rigid canted boom stops affixed to the base to engage downward-tending booms, thereby providing locking of the lift in a raised, over-center position. To lower the lift from the over-center position, '914 requires that the actuator be double-acting, providing expanding actuation for raising, as in the prior art, but also providing contracting actuation for moving the lift back over center for lowering, as provided preferably by a double-acting hydraulic cylinder. '914 teaches connection of the actuator between a cross member of the frame and a cross member of a pair of booms intermediate of the ends of the booms, just as in Michaelsen, limiting the extent to which the lift may be lowered.
Based upon the innovation of '914, U.S. Pat. No. 5,908,264 to Hey teaches a similar over-center boat lift with a locking means for the over-center raised position comprising a rigid canted diagonal member, affixed to the base, that braces against the boom in its over-center position. Hey claims to improve upon '914 in achieving a lower profile by employing a different actuator arrangement. In Hey, the frame extends beyond the point of connection of that boom pair which, in the lowered position, forms an obtuse angle in the parallelogram. The base of the actuator is pivotally connected to a transverse beam disposed on that extended portion of the frame. The transverse beam is so formed that the point of connection of the base of the actuator is lower than the plane of the frame. By this design, even when the parallelogram is fully collapsed, there is a vertical component between the cylinder base and the distal end of the plunger, thereby enabling lift operation from a lower profile in the fully lowered position.
Hey's actuator arrangement, while permitting a lower profile watercraft lift, does not overcome a shortcoming of all the prior art. In all the prior art, including Hey, there is a load spike on the actuator at the lower end of the lift position. In Michaelsen and in '914, this load spike is very pronounced because the vertical component between the actuator base and its distal end is small at the lift's lowered position. Even in Hey, though, the vertical component is smaller at the lift's lowered position than it is as the lift is raised, resulting in a load spike on the actuator from the lowered position. Typically, the load on such prior art actuator arrangements spikes from up to 2,800 pounds per square inch from the lowered position down to about 1,700 pounds per square inch as the lift approaches its raised position.
The highest load on an actuator determines the actuator cylinder diameter, the maximum fluid pressure and the resultant actuator mechanism tolerances necessitated by such pressure. Lowering the maximum load on an actuator has the desirable effects of lowering the size of required actuator cylinder and reducing the tolerances required for actuator operation.
Despite their advantageous use of gravity to stop the lift in a raised position, prior art over-center watercraft lifts, such '914 and Hey, teach a stop means with a significant shortcoming. Such prior art stop means employ a fixed stop of hard, rigid material directly or indirectly engaging the pivoting booms when the lift has reached the desired over-center position. The resultant engagement is jarring because the stop mechanism materials are inelastic. Such jarring engagement is undesirable not only because it creates a less than optimal user experience, but also because jarring subjects the lift apparatus to excessive stain with resultant fatigue, shortening equipment life.
Yet a further shortcoming of the over-center prior art arises from the fact that the stop mechanism in such art restrains the raised, over-center lift with the watercraft support at a fixed height above the lift base. However, a user may have any of a number of reasons for wishing to vary the height of the lift's watercraft support. For example, because of variations over time in the depth of the body of water in which a given lift is employed (e g. ocean tides, annual variations in lake depth, etc.), with the prior art over-center lifts, the height of the raised watercraft above the water's surface may vary considerably, according to variations over time in the depth of the water. A user may wish to vary the height of the watercraft support of the raised lift above the lift's base to accommodate varying water depths. In other cases, a user may wish to vary the height of the lift's watercraft support above the surface of the water for specific watercraft maintenance or transportation purposes. In any case, it is a shortcoming that the prior art presents no means whereby a user can vary the height of the watercraft support of a raised over-center lift.